// Last Update:2015-01-27 22:26:09
/**
 * @file gateway_ip.c
 * @brief 
 * @author 
 * @version 0.1.00
 * @date 2015-01-27
 */
#include <arpa/inet.h>  //for in_addr
#include <linux/rtnetlink.h>    //for rtnetlink
#include <net/if.h> //for IF_NAMESIZ, route_info
#include <stdio.h>
#include <stdlib.h> //for malloc(), free()
#include <string.h> //for strstr(), memset()

#include <sys/types.h>
#include <unistd.h>

#define BUFSIZE 8192

struct route_info
{
	u_int dstAddr;
	u_int srcAddr;
	u_int gateWay;
	char ifName[IF_NAMESIZE];
};

int readNlSock(int sockFd, char *bufPtr, int seqNum, int pId)
{
	struct nlmsghdr *nlHdr;
	int readLen = 0, msgLen = 0;
	do{
		//收到内核的应答
		if((readLen = recv(sockFd, bufPtr, BUFSIZE - msgLen, 0)) < 0)
		{
			perror("SOCK READ: ");
			return -1;
		}
		nlHdr = (struct nlmsghdr *)bufPtr;
		//检查header是否有效
		if((NLMSG_OK(nlHdr, readLen) == 0) || (nlHdr->nlmsg_type == NLMSG_ERROR))
		{
			perror("Error in recieved packet");
			return -1;
		}
		if(nlHdr->nlmsg_type == NLMSG_DONE)
		{
			break;
		}
		else
		{
			bufPtr += readLen;
			msgLen += readLen;
		}
		if((nlHdr->nlmsg_flags & NLM_F_MULTI) == 0)
		{
			break;
		}
	} while((nlHdr->nlmsg_seq != seqNum) || (nlHdr->nlmsg_pid != pId));
	return msgLen;
}

//分析返回的路由信息
void parseRoutes(struct nlmsghdr *nlHdr, struct route_info *rtInfo,char *gateway, char *ifName)
{
	struct rtmsg *rtMsg;
	struct rtattr *rtAttr;
	int rtLen;
	char *tempBuf = NULL;
	struct in_addr dst;
	struct in_addr gate;

	tempBuf = (char *)malloc(100);
	rtMsg = (struct rtmsg *)NLMSG_DATA(nlHdr);
	// If the route is not for AF_INET or does not belong to main routing table
	//then return.
	if((rtMsg->rtm_family != AF_INET) || (rtMsg->rtm_table != RT_TABLE_MAIN))
		return;

	rtAttr = (struct rtattr *)RTM_RTA(rtMsg);
	rtLen = RTM_PAYLOAD(nlHdr);
	for(;RTA_OK(rtAttr,rtLen);rtAttr = RTA_NEXT(rtAttr,rtLen))
	{
		switch(rtAttr->rta_type)
		{
			case RTA_OIF:
				if_indextoname(*(int *)RTA_DATA(rtAttr), rtInfo->ifName);
				break;
			case RTA_GATEWAY:
				rtInfo->gateWay = *(u_int *)RTA_DATA(rtAttr);
				break;
			case RTA_PREFSRC:
				rtInfo->srcAddr = *(u_int *)RTA_DATA(rtAttr);
				break;
			case RTA_DST:
				rtInfo->dstAddr = *(u_int *)RTA_DATA(rtAttr);
				break;
		}
	}
	dst.s_addr = rtInfo->dstAddr;
	if (strstr((char *)inet_ntoa(dst), "0.0.0.0"))
	{
		sprintf(ifName, "%s", rtInfo->ifName);
		printf("ifName: %s\n", ifName);
		//printf("oif:%s",rtInfo->ifName);
		gate.s_addr = rtInfo->gateWay;
		sprintf(gateway, "%s", (char *)inet_ntoa(gate));
		//printf("%s\n",gateway);
		gate.s_addr = rtInfo->srcAddr;
		//printf("src:%s\n",(char *)inet_ntoa(gate));
		gate.s_addr = rtInfo->dstAddr;
		//printf("dst:%s\n",(char *)inet_ntoa(gate));
	}
	free(tempBuf);
	return;
}

int get_gateway(char *gateway, char *ifName)
{
	struct nlmsghdr *nlMsg;
	struct rtmsg *rtMsg;
	struct route_info *rtInfo;
	char msgBuf[BUFSIZE];

	int sock, len, msgSeq = 0;

	if((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0)
	{
		perror("Socket Creation: ");
		return -1;
	}
	memset(msgBuf, 0, BUFSIZE);


	nlMsg = (struct nlmsghdr *)msgBuf;
	rtMsg = (struct rtmsg *)NLMSG_DATA(nlMsg);


	nlMsg->nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg)); // Length of message.
	nlMsg->nlmsg_type = RTM_GETROUTE; // Get the routes from kernel routing table .

	nlMsg->nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST; // The message is a request for dump.
	nlMsg->nlmsg_seq = msgSeq++; // Sequence of the message packet.
	nlMsg->nlmsg_pid = getpid(); // PID of process sending the request.


	if(send(sock, nlMsg, nlMsg->nlmsg_len, 0) < 0){
		printf("Write To Socket Failed…\n");
		return -1;
	}


	if((len = readNlSock(sock, msgBuf, msgSeq, getpid())) < 0) {
		printf("Read From Socket Failed…\n");
		return -1;
	}
	rtInfo = (struct route_info *)malloc(sizeof(struct route_info));
	for(;NLMSG_OK(nlMsg,len);nlMsg = NLMSG_NEXT(nlMsg,len)){
		memset(rtInfo, 0, sizeof(struct route_info));
		parseRoutes(nlMsg, rtInfo, gateway, ifName);
	}
	free(rtInfo);
	close(sock);
	return 0;
}

int main()
{
	char buff[256], ifName[12];
	get_gateway(buff, ifName);
	printf("interface:%s\ngetway: %s\n",ifName, buff);
	return 0;
}
